The present invention relates to a sealing arrangement, to a conveying device, in particular a gear wheel pump, having a sealing arrangement and also to a method for operating the sealing arrangement.
Many polymers as a molten mass are highly reactive upon contact with oxygen. Even extremely small quantities of oxygen can cause reactions which have undesirable effects on the polymer properties. Moreover, it is possible that both the quality of the polymer produced is significantly impaired and also penetrating oxygen can have the effect that the entire reaction chemistry in the reactor vessel is tipped. As a consequence of this, the polymerization process has to be stopped until the vessel and possible supply lines and also conveying devices are cleaned completely.
During the production of polymers in polymerization reactors, in particular, the exclusion of oxygen from the polymerization process and the plant components required therefor is therefore of utmost importance.
It is often the case that gear wheel pumps are used as conveying devices beneath polymerization reactors. The task of these gear wheel pumps is to discharge the finished reaction product from the reactor and to convey it into further plant components.
To avoid contact between oxygen and the conveying medium, it has already been proposed to provide a sheathing or enclosure of the critical (in any case non-tight) process components (e.g. the gear wheel pump) with an additional protective housing. To this end, the relevant plant components are surrounded with a sheet metal casing, for example, the intermediate space between the plant components and the protective housing being flooded with high-purity nitrogen.
If this method is implemented correctly, although the admission of oxygen to possible leakage points of the process components (e.g. gear wheel pump) is avoided, the known method has significant disadvantages:                The sheathing of plant components is very expensive in production.        The sheathed plant component is no longer easily accessible, and therefore can only be observed poorly and maintained only once the sheathing has been disassembled or opened.        When a complete conveying device is sheathed, it is also necessary for the drive to be sheathed, otherwise a significant leakage of nitrogen occurs in the region of the rotating shaft, since the latter can be sealed off poorly.        Leadthroughs for sensors and actuators (e.g. hand wheels and adjustment mechanisms which can be operated from the outside) can only be sealed off very poorly.        The known sheathings do not afford an optimum seal, and therefore large quantities of high-purity nitrogen often escape unnecessarily.        Moreover, there is a risk of suffocation for operating personnel in the area of a non-tight sheathing filled with pure nitrogen, because the oxygen-containing ambient air is displaced by nitrogen.        
In order to seal off separating points which are required within plant components (firstly against the escape of polymer from the conveying device at a high internal pressure or else against possible admission of oxygen), the widest variety of sealing variants are used.
By way of example, sealing points between a gear wheel pump housing and a gear wheel pump cover are sealed off by incorporating gas-filled metal O rings in the cover or the housing.
Under normal conditions, these afford a fairly reliable seal against the escape of polymer from the pump at a high internal pressure. However, the sealing action is not optimal for gases under all operating states.
It is also known in the prior art that individual seals are superposed with nitrogen from the outside and the latter then flows outward into the environment via gaps. In this solution, too, large quantities of nitrogen are wasted. Moreover, the system as a whole cannot be controlled.